The present invention generally relates to detergent compositions and additives used in those compositions. Specifically, the present invention relates to compositions containing .alpha.-sulfofatty acid esters that can be additives to a detergent, and methods for making and using the same.
Detergent compositions have been known and used as cleaning agents for many years. See U.S. Pat. Nos. 5,783,540, 5,133,892, 4,219,435, 5,358,655, 4,913,832, 5,324,649, 5,397,494, 4,828,745, 4,588,080, 4,518,516, 5,482,641, 4,569,780, 4,828,749, Re. 32,763, and Re. 32,818, the disclosures of which are incorporated herein by reference. A large number of surfactants have been used with detergents because of their excellent surface-activity characteristics. Recently, interest in .alpha.-sulfofatty acid esters (also referred to hereafter as "sulfofatty acids") has increased due to the superior cleaning properties of these compounds. For example, see U.S. Pat. Nos. 5,681,803, 4,816,188, 5,688,982, 5,475,134, and 5,429,773, the disclosures of which are incorporated herein by reference.
.alpha.-Sulfofatty acid esters are typically manufactured as sulfofatty acid salts. These salts also exhibit surface active agent properties. Sulfofatty acid salts can be a mixture of salt forms, typically including mono- and di-salts. For example, methyl ester sulfonate ("MES") has both mono- and di-salt forms, i.e. mono-sodium MES ("msMES") and di-sodium MES ("dsMES"). Mono-salts of sulfofatty acids generally have superior washing properties as compared with the di-salt forms. The mono-salts are unstable, however, and react with alkali-containing detergent components to form di-salts. In particular, di-salts form under alkaline conditions at pH values of 9 or greater. For example, mono-sodium methyl ester sulfonate reacts with caustic soda (NaOH) to form a di-salt by the following chemical reaction: ##STR1## Other bases also catalyze di-salt formation. Moisture and/or humid conditions can accelerate base-catalyzed di-salt formation. Heat, in the presence of moisture or humid conditions, can further accelerate di-salt formation.
The formation of di-salts detracts from the desirability of sulfofatty acids as a detergent additive. As the amount of di-salt increases, the amount of the mono-salt correspondingly decreases. To compensate, more mono-salt of the sulfofatty acid must be added to a detergent mixture to provide the same level of surface active agent performance, which increases the cost and unit size of the detergent. Thus, there is a need for compositions, and methods of making and using such compositions, which limit additional di-salt formation in sulfofatty acids.